Since an engine valve, particularly an exhaust valve, used for an internal combustion engine, such as an automotive engine, is exposed to high temperature, the engine valve is configured hollow and put with metallic sodium inside the valve stem thereof. Though metallic sodium to be put is solid at room temperature, it has a melting point of about 98° C. and thus liquefied at about 100° C. of relatively low temperature. Accordingly, when the valve is warmed by activating engine, the metallic sodium become liquid and is shaken in the valve stem owing to a vertical movement of the valve, thereby heat transferred from a combustion chamber to a valve head is conducted through the valve stem, so as to dissipate to a water jacket of a cylinder head through a valve guide contacting with the valve stem. Thus, overheat of the combustion chamber is prevented by cooling of the engine valve. Moreover, since a specific gravity of metallic sodium is 0.97, i.e. less than that of water, the valve loaded with metallic sodium can contribute to light-weighting of the entire valve.
Metallic sodium has a strong reducing action so that it deoxidizes water to generate hydrogen and to change itself to sodium hydroxide. Therefore, in order to prevent it from being oxidized and to stably preserve it for a long period of time, metallic sodium is stored under the condition where it is immersed in an organic solvent, such as kerosene, liquid paraffin (a mixture of relative long-chain saturated hydrocarbons, having a boiling point of several hundred degrees) or the like, with blocking water and air. Besides, each of kerosene and liquid paraffin has a less specific gravity than metallic sodium, so that metallic sodium is securely blocked from water and air without floating on the surface of such solvents.
The valve stem of the engine valve can be loaded with such metallic sodium which is stored in an organic solvent by taking out a bulk body of metallic sodium immersed in the organic solvent from the solvent, melting the metallic sodium, pouring the metallic sodium in a melting state into the stem portion of the engine valve and then cooling the valve, or cutting a bulk body of the metallic sodium into a predetermined size small enough to be loaded into the engine valve and then loading it.
However, a surface of the bulk body of metallic sodium taken out from the organic solvent is coated with kerosene or liquid paraffin. Because such kerosene or liquid paraffin may inhibit a heat transfer of the metallic sodium in the melting state in the engine valve, the bulk body of the metallic sodium is utilized after wiping off such kerosene or liquid paraffin. Further, comparing kerosene with liquid paraffin, the latter contains fewer impurities. Accordingly, use of the bulk body of the metallic sodium immersed in the liquid paraffin more easily achieves to obtain the metallic sodium having higher purity capable of being used for many purposes.
On the surface of a commercially available metallic sodium, some cracks may occur. Melting a bulk body of the metallic sodium with the cracks to form liquid metallic sodium causes inconveniences, for example, contamination by impurities such as kerosene, liquid paraffin. For this reason, in order to obtain only metallic sodium surely, firstly, a quantity of the metallic sodium is determined on the basis of the volume thereof. Secondly, the cracks in the surface of the metallic sodium are removed by cutting out the surface including the cracks. Then, the metallic sodium is utilized. Thus, conventionally, as a result of separate examination of a surface condition of each bulk body of the metallic sodium, some whose surface condition is good are melted and purified after wiping out liquid paraffin, or the like and others which the cracks occur in the surface are melted after cutting out the surface relatively thick. However, this method needs an examination for each metallic sodium ingot and a time-consuming operation of cutting out the surface of the defective metallic sodium. Further, this method has a drawback of reducing a manufacturing yield of purified metallic sodium due to cut-out metallic sodium chips.
Further, the metallic sodium taken out from the organic solvent unavoidably comes in contact with atmospheric oxygen to be oxidized at the surface thereof to form sodium oxide. This is a factor to make it impossible to use such metallic sodium taken from the organic solvent for purposes requiring high purity.
As mentioned above, the metallic sodium loaded into a hollow engine valve vertically moves in the valve stem, so that heat in the combustion chamber is dissipated toward the cylinder head. However, a contamination of the metallic sodium by liquid paraffin or kerosene causes a carbonization of such liquid paraffin or kerosene and inhibition of movement of liquid metallic sodium. That lead to inhibition of a cooling effect, so that the heat of the combustion chamber cannot be sufficiently dissipated. Consequently, a deterioration of fuel consumption and a bad influence on the durability of constituent materials of the engine valve may occur.